A linear structure, usually of earth or gravel, constructed so as to extend above the natural ground surface and designed to hold back water from overflowing a level tract of land, to retain water in a reservoir, tailings in a pond, or a stream in its channel, or to carry a roadway or railroad; e.g., a dike, seawall, or fill.

a. Penetration of microcrystalline groundmass material into phenocrysts, making their normal euhedral boundaries incomplete. An irregular corrosion or modification of the outline of a crystal by the magma from which it previously crystallized or in which it occurs as a foreign inclusion; esp. the deep corrosion into the sides of a phenocryst. The penetration of a crystal by another, generally euhedral, crystal. Such a crystal is called an embayed crystal.

b. A downwarped area containing stratified rocks, either sedimentary or volcanic or both, that extends into a terrain of other rocks, e.g., the Mississippi Embayment of the U.S. Gulf Coast.

a. Sectile, ductile; occurs as gray, yellowish or greenish-gray hornlike masses, waxy coatings, or crusts, as a secondary mineral in the oxidized zone of silver deposits; commonly associated with native silver, manganese oxides, and secondary lead and copper minerals.

b. The chief source of silver in some Chilean mines occurring as yellow-green incrustations and masses. It occurs in Australia at Broken Hill, New South Wales, and at Silver Reef, Victoria; widespread in the silver mining districts of the United States. Syn: horn silver.

a. A brilliant green gem variety of beryl, highly prized as a gemstone. The color, which is caused by chromium or vanadium impurity, ranges from medium-light to medium-dark tones of slightly bluish green to slightly yellowish green. Syn: smaragd.

A step cut in which the finished gem is square or rectangular and the rows (steps) of elongated facets on the crown and pavilion are parallel to the girdle with sets on each of the four sides and in some cases at the corners; commonly used on diamonds to emphasize the absence of color and on emeralds and other colored stones to enhance the color. CF: step cut.

The trade name for a color filter through which genuine emeralds and some other genuine stones appear reddish to violetish while glass imitations and some genuine stones appear greenish. Syn: beryloscope. See also: Walton filter.

An assembled stone commonly consisting of a crown and pavilion of rock crystal bound together by transparent green cement or a thin piece of green sintered glass; immersed in water and viewed sideways, the top and bottom are colorless with a line of color along the girdle. Green or colorless beryl may be used for the crown and possibly for the pavilion. Glass may be used for the pavilion, and sometimes for the crown as well, but the trade still calls it an emerald triplet. Syn: Soude emerald. See also: tripletine. CF: triplet.

a. A change in the levels of water and land such that the land is relatively higher and areas formerly under water are exposed; it results either from an uplift of the land or from a fall of the water level. Ant: submergence.

b. The point where an underground stream appears at the surface to become a surface stream. Syn: resurgence; rise; rising.

A granular rock that is composed essentially of an impure mixture of corundum, magnetite, and spinel, and that may be formed by magmatic segregation or by metamorphism of highly aluminous sediments. Syn: emery; corundolite.

A chute with narrow opening for the cleaning of coal. The slate, traveling slowly because of friction, falls into the openings and thus is removed from the coal, which, rolling freely down the incline, is carried over the narrow gap.

a. A light, hand-operated churn drill for testing placers from 10 to 125 ft (3.0 to 38.1 m) deep, though it is more commonly used for shallower holes. It consists of a string of 4-in (10.2-cm) casing, to the lower end of which is screwed a toothed cutting shoe. To the upper part, projecting above the ground, is fastened a round steel platform on which workers stand while operating the drilling tools. The casing can be turned by workers or a horse on the end of a long sweep fastened to the platform. The core of material inside the casing is loosened and brought to the surface by a drill pump on the end of a string of rods. CF: Banka drill.

In bituminous coal mining, a laborer who pulls empty cars from cage or detaches them from hoisting cable when hoisting of loaded cars is done on one side of the shaft or haulage slope and lowering is done on the other.

Having crystal forms that, while possessing neither a plane nor a center of symmetry, may occur in two positions that are mirror images of one another. The two positions cannot be converted into each other by any rotation, but are related to each other as are the right and left hand, hence designated right- and left-handed forms. Enantiomorphous crystals cause circular polarization of light, e.g., quartz.

Cut in a style characterized by a smooth-domed, but unfaceted, surface; e.g., a ruby cut en cabochon in order to bring out the star. Etymol: French. See also: cabochon. Commonly used for garnets (carbuncles) and for those gems that depend for their beauty largely upon minute oriented inclusions (e.g., crocidolite, star ruby, or sapphire), the plan of the stone being circular or oval.

A mechanically operated, sloping table by which heavy and light materials are separated. The end motion imparted to the table tends to drive all minerals up the slope of the table, but a flow of water carries the light materials down faster than the mechanical motion carries them up. The heavy materials settle to the bottom and finally reach the upper end and are delivered into a proper receptacle. The Gilpin County, Imlay, and Golden Gate concentrators are the chief types. Syn: Imlay table.

a. Gate at the front end of a car as it travels toward the dump. This gate has hooks that are engaged at the dump by stirrups that lift it, so that when the dump pitches forward the coal slides under the uplifted endgate and is discharged onto a chute or over a dump pile.

b. A gate leading to and at right angles to an end face. Also called ending.

A device for hauling coal in which a chain passes from the engine along one side of the road, around a pulley at the far end, and back again on the other side of the road. Empty cars, attached to one side of the chain by various kinds of clips or hooks, are hauled into the mine; loaded cars attached to the other side of the chain are hauled out of the mine.

The boundary lines of a mining claim that cross the general course of the vein at the surface. If the side lines cross the course of the vein instead of running parallel with it, they then constitute endlines. When a mining claim crosses the course of the lode or vein instead of being along such lode or vein, the endlines are those that measure the width of the claim as it crosses the lode.

Extralateral rights are allowed on a claim whose endlines converge, but they are not allowed in case the endlines diverge. Converging endlines on a claim would have the disadvantage of giving the owner of such a claim a continually diminishing length of vein on working down the dip.

a. One of the two or more simple compounds of which an isomorphous (solid-solution) series is composed. For example, the end members of the plagioclase feldspar series are albite, NaAlSi (sub 3) O (sub 8) , and anorthite, CaAl (sub 2) Si (sub 2) O (sub 8) . Syn: minal.

b. One of the two extremes of a series; e.g., types of sedimentary rock or of fossils.

Derived from within; said of a geologic process, or of its resultant feature or rock, that originates within the Earth, e.g., volcanism, volcanoes, extrusive rocks. The term is also applied to chemical precipitates, e.g., evaporites, and to ore deposits that originate within the rocks that contain them. CF: exogenetic; hypogene. Syn: endogene; endogenic; endogenous.

Changes within an igneous rock produced by the complete or partial assimilation of country-rock fragments or by reaction upon it by the country rock along the contact surfaces. It is a form of contact metamorphism with emphasis on changes produced within the igneous body rather than in the country rock. The term was originated by Fournet in 1867. CF: exomorphism. Partial syn: endogenetic effects. Syn: endometamorphism; endomorphic metamorphism.

In gemology, an instrument that affords a magnified image of the drill hole of a pearl, used to distinguish between genuine and cultured pearl. A tiny beam of light is directed into the walls of the drill hole to reveal whether the structure of the pearl's core is concentric (genuine) or parallel (cultured).

a. The brick, concrete, or stonework construction at the sides of an excavation built to carry a flat or arched roof. Also called sidewall.

b. The vertical refractory wall, farthest from the furnace chamber, of the downtake of an open-hearth steel furnace. c. One of the two vertical walls terminating a battery of coke ovens or a bench of gas retorts; it is generally constructed of refractory bricks and heat-insulating bricks with an exterior facing of building bricks.

Said of geologic features that are in an overlapping or staggered arrangement, e.g., faults. Each is relatively short, but collectively they form a linear zone, in which the strike of the individual features is oblique to that of the zone as a whole. Etymol: French en echelon, in steplike arrangement.

b. The capacity for producing motion. Energy holds matter together. It can become mass, or it can be derived from mass. It takes such forms as kinetic, potential, heat, chemical, electrical, and atomic energy, and it can be changed from one of these forms to another. c. Kinetic energy is that due to motion, and potential energy is that due to position. In a stream, for example, the total energy at any section is represented by the sum of its potential and kinetic energies.

Geology as applied to engineering practice, esp. mining and civil engineering. As defined by the Association of Engineering Geologists (1969), it is the application of geologic data, techniques, and principles to the study of naturally occurring rock and soil materials or ground water for the purpose of ensuring that geologic factors affecting the location, planning, design, construction, operation, and maintenance of engineering structures, and the development of ground-water resources, are properly recognized and adequately interpreted, utilized, and presented for use in engineering practice. Syn: geologic engineering.

a. A system of rope haulage in which the loads are raised or lowered on the slope by a steam or electric hoist. In the simplest form only one track and one rope are required, and power is used for raising the load. Double engine planes have two separate tracks or three rails and a passing turnout.

b. A roadway, horizontal or inclined, on which tubs or cars are hauled by rope haulage. c. Direct rope haulage.

A method of refining silver in which a small reverberatory furnace with a movable bed and a fixed roof is used. The bullion is charged gradually, and the silver is refined in the same furnace where the cupellation is carried on.

A method of smelting lead ore in which the characteristics are a large charge of lead ore, a quick roasting, a high temperature throughout, and the aim to extract all the lead in the reverberatory. The hearth inclines toward the middle of one of the sides, the lead collects in the furnace and is tapped at intervals into an outside kettle.

A conveyor comprising a series of skeleton or solid flights on an endless chain or other linkage that operates in horizontal, inclined, or vertical paths within a closely fitted casing for the carrying run. The bulk material is conveyed and elevated en masse in a substantially continuous stream with a full cross section of the casing. Also called chain conveyor. Syn: continuous stream conveyor.

A fuse lighter similar to a Fourth-of-July sparkler, that burns for 2-1/2 min, sufficient time to light 30 to 50 fuses. The lead splitter is a lead tube of diameter about 1/8 in (3.2 mm) in filled with a slow-burning powder that burns at the rate of 36 s/ft (118 s/m) with a hot splitting flame.

A shell, similar to a shotgun cartridge, that contains an ignition compound in the base. As many as seven fuses can be pushed into the shell until the fuses contact the ignition compound. The lighting of one fuse, which burns into the shell, sets off the compound and ignites the other six fuses.

a. A measure of the unavailable energy in a system; i.e., energy that cannot be converted into another form of energy.

b. A measure of the mixing of different kinds of sediment; high entropy is approach to unmixed sediment of one kind. c. Ratio of amount of heat added to air to the absolute temperature at which it is added. Measured in Btu. d. Specific entropy is the ratio of entropy to weight of substance.

b. An underground passage used for haulage or ventilation, or as a manway. Back entry, the air course parallel to and below an entry. Distinguished from straight entry, front entry, or main entry. Dip entry, an entry driven downhill so that water will stand at the face directly down a steep dip slope. Gob entry, a wide entry with a heap of refuse or gob along one side. Slab entry, an entry that is widened or slabbed to provide a working place for a second miner. Double entry, a system of opening a mine by two parallel entries; the air current is brought into the rooms through one entry and out through the parallel entry or air course. Cutoff entry, an entry driven to intersect another and furnish a more convenient outlet for the coal. Single entry, a system of opening a mine by driving a single entry only, in place of a pair of entries. The air current returns along the face of the rooms, which must be kept open. Triple entry, a system of opening a mine by driving three parallel entries for the main entries. Twin entry, a pair of entries close together and carrying the air current in and out, so laid out that rooms can be worked from both entries. Also called double entry. c. A coal heading. To develop a coal mine in the United States, one or more sets of main entries are driven into the take. Each set consists of four to eight coal headings, connected at intervals by crosscuts. From these, and usually at right angles, butt entries, three to six in number, are driven at intervals of up to 1,500 yd (1.37 km). Between the sets of butt entries, face entries, three to four in number, are driven at intervals of up to 500 yd (0.46 km) to form a block or panel. The entries to split the panels may be 12 to 20 ft (3.7 to 6.1 m) wide and at 50- to 100-ft (15.2- to 30.5-m) centers. Each entry is made as productive as possible, and productivity is often higher in the entry work than in pillar extraction. See also: pillar-and-stall.

A combination mining machine designed and built to work in entries and other narrow places, and to load coal as it is broken down. An undercutting frame and two vertical shearing frames serve to undercut and shear the sides of the coal, so that the ram equipped with bars and operated by hydraulic jacks can break down the coal. The height at which the ram operates against the coal, when the undercut and shearing are completed, is adjustable. A conveyor in the undercutting frame carries the broken-down coal back to another conveyor mounted on a turntable so that the coal can be loaded into a mine car, or slate can be deposited on the gob side of the entry. The entire machine is mounted in a pan.

In bituminous coal mining, one who operates a type of coal cutter known as a heading machine that is adapted to the driving of underground haulageways in coal from one part of the mine to another or to the surface. Also called entry driving machine operator.

b. One who enters upon public land with intent to secure an allotment under homestead, mining, or other laws. c. In anthracite and bituminous coal mining, one who is engaged in driving a haulageway, airway, or passageway from one place to another in the mine or to the surface. Also called heading driver.

Pillars of coal left in the mouths of abandoned rooms to support the road, entry, or gangway until the entry pillars are drawn. In Arkansas, these pillars are called entry stumps even when the rooms are first driven, before any pillars are pulled or the rooms abandoned.

An analysis of environmental conditions which may involve baseline environmental analyses and data gathered with regard to zoological, botanical, geologic, and economic factors. This data may be utilized for environmental impact statements. Abbrev.: EA.

An evaluation of environmental conditions at a particular facility or site. Major items that could be relevant to an environmental audit for a mining facility may include information on permits, surface and mineral rights, mine ownership and violations, archaeological sites, hydrology issues, air pollution, waste disposal, impoundments, mine fires, underground injections and previously mined areas.

A statement which is prepared by a Federal agency with regard to a permit, and is required under the National Environmental Policy Act (NEPA). The EIS may include but is not limited to information relating to the purposes and needs to which the agency is responding by the preparation of the EIS, alternatives, and the environmental consequences which may arise from the proposed action.

As defined by Gilbert (1890), a form of diastrophism that has produced the larger features of the continents and oceans, for example, plateaus and basins, in contrast to the more localized process of orogeny, which has produced mountain chains. Epeirogenic movements are primarily vertical, either upward or downward, and have affected large parts of the continents, not only in the cratons but also in stabilized former orogenic belts, where they have produced most of the present mountainous topography. Some epeirogenic and orogenic structures grade into each other in detail, but most of them contrast strongly. Adj. epeirogenic. Syn: epeirogenesis.

A bottom sampler consisting of a pair of sheet-metal skis attached to a light framework for a silk or nylon net. Removable rakers in front of the net stir up the bottom as the dredge advances, permitting the net to capture the benthic fauna and flora contained in the sediment. A bottom-walking wheel connected to a small counter indicates the distance over the bottom the device travels during a haul.

A metamorphosed gabbro or diabase in which generally fibrous amphibole (uralite) has replaced the original clinopyroxene (commonly augite). It is usually massive but may have some schistosity. See also: epidiabase.

The hydrothermal introduction of epidote into rocks or the alteration of rocks in which plagioclase is albitized, freeing the anorthite molecule for the formation of epidote and zoisite, often accompanied by chloritization. These processes are characteristically associated with metamorphism.

a. Said of a mineral deposit formed later than the enclosing rocks. CF: syngenetic.

b. Produced on or near the Earth's surface, e.g., epigenetic valleys, etc. c. In ore petrology, applied to mineral deposits of later origin than the enclosing rocks or to the formation of secondary minerals by alteration. Syn: epigenic.

a. Induced orientation of the crystal lattice of an electrodeposit at the plane of contact with the undisturbed underlying metal.

b. Orientation of one crystal with that of the crystalline substrate on which it grew; e.g., halite growing on a cleavage plane of mica because the mesh of the net of halite nearly coincides in shape and size with the pseudohexagonal net of the mica substrate. Adj: epitactic, epitaxic, epitaxial. CF: distaxy; topotaxy; syntaxy.

Said of a hydrothermal mineral deposit formed within about 1 km of the Earth's surface and in the temperature range of 50 to 200 degrees C, occurring mainly as veins. Also, said of that depositional environment. CF: hypothermal deposit; mesothermal; leptothermal; telethermal; xenothermal.

According to Grubenmann's classification of metamorphic rocks (1904), the uppermost depth zone of metamorphism, characterized by low to moderate temperatures (less than 300 degrees C) and hydrostatic pressures with low to high shearing stress. Modern usage stresses pressure-temperature conditions (low metamorphic grade) rather than the likely depth of zone. CF: mesozone; katazone.

An unsheathed explosive incorporating cooling agents, which is equivalent in safety (relating to the ignition of methane-air mixture) on a charge-weight basis to an explosive having a sheath of cooling agents around it. Abbrev. for equivalent-to-sheathed explosive.

a. Said of a crystal having the same or nearly the same diameter in all directions. CF: anisodesmic; tabular; prismatic. Syn: equidimensional; isometric.

b. Said of a sedimentary particle whose length is less than 1.5 times its width. c. Said of a rock in which the majority of grains are equant. d. Refers to crystals with roughly equal dimensions. CF: tabular; lathlike; rodlike; acicular.

a. A perfect balance of physical forces such that when two or more forces act upon a body, the body remains at rest.

b. The state in which a reversible chemical reaction is proceeding at the same rate in each direction. Metastable equilibrium is a steady unsatisfied state that will undergo further change on addition of the phase necessary to complete its stability. Physical equilibrium can connote stable coexistence of a substance in two or more phases, such as solid, liquid, and/or vapor. c. In geology, a balance between form and process, e.g., between the resistance of rocks along a coast and the erosional force of the waves. d. That state of a chemical system in which the phases do not undergo any change of properties with the passage of time, provided they have the same properties when the same conditions are again reached by a different procedure.

The moisture content retained at equilibrium in an atmosphere over a saturated solution of potassium sulfate at 30 degrees C, and 96% to 97% relative humidity. When the sample, before such equilibrium, contains total moisture at or above the equilibrium moisture, the equilibrium moisture may be considered as equivalent to inherent or bed moisture, and any excess may be considered as extraneous moisture.

A technique used in electrical prospecting requiring artificial currents. It is based on the principle that if two electrodes are inserted in the ground and an external voltage is applied across them, there will be a flow of current through the earth from one electrode to the other. If the medium through which the current flows is homogeneous in its electrical properties, the flow lines will be regular and in a horizontal plane, symmetrical about the line joining the electrodes. Any inhomogeneities in these properties will cause distortions in the lines of current flow. Such distortions indicate the existence of buried material with either higher conductivity than its surroundings, so that it attracts the flow lines toward itself, or with lower conductivity, so that it tends to force the lines into the surrounding medium.

A surface on which the potential is everywhere constant for the attractive forces concerned. The gravity vector is everywhere normal to a gravity equipotential surface; the geoid is an equipotential. Syn: gravity equipotential surface; niveau surface; level surface.

Corresponding in geologic age or stratigraphic position; esp. said of strata or formations (in regions far from each other) that are contemporaneous in time of formation or deposition or that contain the same fossil forms. n. A stratum that is contemporaneous or equivalent in time or character.

a. The diameter of a hypothetical sphere composed of material having the same specific gravity as that of the actual particle and of such size that it will settle in a given liquid at the same terminal velocity as the actual particle. Also called equivalent size.

The quantity of water that would be evaporated by a given apparatus if the water is received by the apparatus at 212 degrees F (100 degrees C), and vaporized at that temperature under atmospheric pressure. It is expressed in kilograms per hour.

The resistance of a mine airway obstruction, duct or pipe elbow, valve, damper, orifice, bend, fitting, or other obstruction to flow, expressed in the number of feet of straight airway, duct, or pipe of the same cross section that would have the same resistance.

A concept used in evaluating the size of fine particles by a sedimentation process; it is defined as the diameter of a sphere that has the same density and the same freefalling velocity in any given fluid as the particle in question. CF: particle size. Syn: equivalent freefalling diameter.

a. The radius of a spherical particle of density 2.65 (the density of quartz) which would have the same rate of settling as the given particle.

b. A measure of particle size, equal to the computed radius of a hypothetical sphere of specific gravity 2.65 (quartz) having the same settling velocity and same density as those calculated for a given sedimentary particle in the same fluid; one half of the equivalent diameter.

A composite of mean radiant temperature and air temperature; also defined as the mean temperature of the environment effective in controlling the rate of sensible heat loss from a black body in still air when the surface temperature and size of the black body are comparable to those of the human body. Where the enclosure surface (mean radiant temperature) and air temperatures are equal, this temperature is also the British equivalent temperature; when not equal, the British equivalent temperature is that temperature at which a body with an 80 degrees F (26.7 degrees C) surface temperature will lose sensible heat at the same rate as in the given environment. Syn: British equivalent temperature.

The formal geochronologic unit next in order of magnitude below an eon, during which the rocks of the corresponding erathem were formed; e.g., the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. Each of these includes two or more periods, during each of which a system of rocks was formed. Long-recognized Precambrian Eras are the Archeozoic (older) and Proterozoic (younger).

The group of physical and chemical processes by which earth or rock material is loosened or dissolved and removed from any part of the Earth's surface. It includes the processes of weathering, solution, corrosion, and transportation. The mechanical wear and transportation are effected by rain, running water, waves, moving ice, or winds, which use rock fragments to pound or to grind other rocks to powder or sand.

a. A land surface shaped and subdued by the action of erosion, esp. by running water. The term is generally applied to a level or nearly level surface.

b. An area that has been flattened by subaerial or marine erosion to form an area of relatively low relief at an elevation close to the base level (sea level) existing at the time of its formation. Relics of such surfaces may now be found far above sea level owing to the falling base level, below the present ocean surface.

A rock fragment carried by glacial ice or by floating ice, deposited at some distance from the outcrop from which it was derived, and generally though not necessarily resting on bedrock of different lithology. Size ranges from a pebble to a house-size block.

a. Of a traverse, the amount by which the computed position of the last point of the traverse fails to coincide with the initial point; i.e., the length of line necessary to close the traverse. Frequently, also, the ratio of the linear error of closure to the perimeter (also known as the error of the survey).

b. Of angles, the amount by which the sum of the measured angles fails to equal the true sum. c. Of azimuths, the amount by which the measurement of the azimuth of the first line of a traverse, made after completing the circuit, fails to equal the initial measurement. d. Of a level circuit, the amount by which the last computed elevation fails to equal the initial elevation; or the amount by which the differences of elevation in a circuit fail to add up (algebraically) to zero. e. Of a horizon, the amount by which the sum of the angles measured around the horizon differs from 360 degrees . f. Of a triangle, the amount by which the sum of the three angles of a triangle differs from the true sum; i.e., 180 degrees plus the spherical excess.

a. A long, more or less continuous cliff or relatively steep slope facing in one general direction, breaking the continuity of the land by separating two level or gently sloping surfaces, and produced by erosion or by faulting. The term is often used synonymously with scarp, although escarpment is more often applied to a cliff formed by differential erosion.

b. A steep, abrupt face of rock, often presented by the highest strata in a line of cliffs, and generally marking the outcrop of a resistant layer occurring in a series of gently dipping softer strata; specif. the steep face of a cuesta. CF: cuesta.

A classifier of the free-settling type in which the settled material is removed by dragging it up an inclined plane by means of a continuous belt of flat blades or paddles. It is continuous in its operation.

A conglomerate or breccia with rapid lateral passage through grit to fine sandstone; cement usually ferruginous with some lime and alumina. Characteristically developed amid variegated clays of Etruria Marl group of Upper Coal Measures in the English Midlands.

An alkali gabbro primarily composed of plagioclase, hornblende, biotite, and titanaugite, with lesser amounts of alkali feldspar and nepheline. Essexite grades into theralite with a decrease in potassium feldspar and an increase in the feldspathoid minerals. Its name is derived from Essex County, MA.

A sedimentary deposit laid down in the brackish water of an estuary, characterized by fine-grained sediments (chiefly clay and silt) of marine and fluvial origin mixed with a high proportion of decomposed terrestrial organic matter; it is finer grained and of more uniform composition than a deltaic deposit.

The angle formed between the true horizon and the actual plane of the etch ring in an acid bottle as measured before capillarity corrections. Also called apparent angle. See also: capillarity. CF: apparent dip.

A marking, commonly in the form of minute pits, produced by a solvent on a crystal surface; the form varies with the mineral species and the solvent, but reflects the symmetry of the structure; also called etching figure.

A method, using a soda-lime glass tube partially filled with a dilute solution of hydrofluoric acid, of determining the angle at which a borehole is inclined at any specific point of its course below the collar. See also: acid-dip survey.

Regular surface marking developed by solvent action on smooth surface of alloy or crystal, and characteristic for that specific substance. The reagent used is an etchant, usually of an acid in water or alcohol.

The time required for a dilute solution of hydrofluoric acid of a specific strength to etch the inside of an acid bottle enough so that the line of demarcation between the etched and unetched portions of the acid bottle is clearly discernible. Also known as etching time.

b. A dark-colored extrusive rock intermediate in composition between leucitite and nephelinite with phenocrysts of clinopyroxene in a dense groundmass of leucite, nepheline, and clinopyroxene. The name is not included in the IUGS classification of extrusive igneous rocks.

Pertaining to worldwide changes of sea level that affect all the oceans. Eustatic changes may have various causes, but the changes dominant in the last few million years were caused by additions of water to, or removal of water from, the continental icecaps.

Said of the banded structure of certain extrusive rocks, which results in a streaked or blotched appearance. Also, said of a rock exhibiting such structure, e.g., a eutaxite. The bands or lenses were originally ejected as individual portions of magmas, were drawn out in a viscous state, and formed a heterogeneous mass in response to welding.

Said of a system consisting of two or more solid phases and a liquid whose composition can be expressed in terms of positive quantities of the solid phases, all coexisting at an (isobarically) invariant point, which is the minimum melting temperature for the assemblage of solids. Addition or removal of heat causes an increase or decrease, respectively, of the proportion of liquid to solid phases, but does not change the temperature of the system or the composition of any phases. See also: eutectoid.

The ratio of solid phases crystallizing from the eutectic liquid at the eutectic temperature. It is such as to yield a gross composition for the crystal mixture that is identical with that of the liquid. It is most frequently stated in terms of weight percent.

A pattern of intergrowth of two or more minerals, formed as they coprecipitate during crystallization, e.g., the quartz and feldspar of graphic granite. See also: exsolution texture. Syn: eutectoid texture.

An orthorhombic mineral, (Y,Ca,Ce,U,La,Th)(Nb,Ta,Ti) (sub 2) O (sub 6) ; forms a series with polycrase; brilliant to vitreous brown to black; in pegmatites and placers commonly with monazite in Canada; Madagascar; Norway; and Pennsylvania. A source of uranium, niobium, and tantalum. Formerly called loranskite.

a. Pertaining to an environment of restricted circulation and stagnant or anaerobic conditions, such as a fjord or a nearly isolated or silled basin with toxic bottom waters. Also, pertaining to the material (such as black organic sediments and hydrogen-sulfide muds) deposited in such an environment or basin, and to the process of deposition of such material (as in the Black Sea).

b. Pertaining to a rock facies that includes black shales and graphitic sediments of various kinds. Etymol: Greek euxenos, hospitable.